We plan to solve the three-dimensional structure of RPTP? by X-ray crystallography. RPTP? belongs to an important family of receptor-type protein tyrosine phosphatases, which are involved in key cellular functions, such as cell growth, proliferation and differentiation. RPTP? (1) comprises a short, highly glycosylated extracelllular domain, a membrane spanning segment, and two intracellular domains (termed D1 and D2). Both D1 and D2 display tyrosine-phosphatase activity, albeit to different levels. We purified the isolated domains D1 and D2, and have succeeded in obtaining crystals for D1 and D2. We have already solved two structures by molecular replacement and we now desperately need higher resolution data.